1. Field of the Invention
The present invention relates to an optical resin film and a polarizing plate and a liquid crystal display device comprising same.
2. Description of the Related Art
Liquid crystal display devices have been widely used for monitor for personal computer and cellular phone, television, etc. because they are advantageous in that they can operate at low voltage with low power consumption and are available in small size and thickness. These liquid crystal display devices have been proposed in various modes depending on the alignment of liquid crystal molecules in the liquid crystal cell. To date, TN mode, in which liquid crystal molecules are aligned twisted at about 90 degrees from the lower substrate to the upper substrate of the liquid crystal cell, has been a mainstream.
A liquid crystal display device normally comprises a liquid crystal cell, an optical compensation sheet and a polarizer. The optical compensation sheet is used to eliminate undesirable coloring of image or expand the viewing angle. As such an optical compensation sheet there is used a stretched birefringent film or a transparent film coated with a liquid crystal. For example, Japanese Patent 3,027,805 discloses a technique for the expansion of the viewing angle involving the application to a TN mode liquid crystal cell of an optical compensation sheet obtained by spreading a discotic liquid crystal over a triacetyl cellulose film, and then orienting and fixing the coat layer. However, liquid crystal display devices for TV use which are supposed to give a wide screen image that can be viewed at various angles have severe requirements for dependence on viewing angle. These requirements cannot be met even by the aforementioned approach. To this end, liquid crystal display devices of modes different from TN mode, including IPS (In-Plane Switching) mode, OCB (Optically Compensatory Bend) mode, VA (Vertically Aligned) mode, have been under study. In particular, VA mode has been noted as liquid crystal display device for TV use because it gives a high contrast image and can be produced in a relatively high yield.
Despite its capability of attaining substantially full black display in the direction along the line normal to panel, VA mode liquid crystal display devices have been disadvantageous in that when the panel is viewed in oblique direction, light leakage occurs, causing the reduction of viewing angle. In order to solve this problem, it has been proposed that a first retardation plate having a positive refractive anisotropy nx>ny=nz and a second retardation plate having a negative refractive anisotropy nx=ny>nz be used in combination to eliminate light leakage (as disclosed in Japanese Patent 3,027,805). It has also been proposed that an optically biaxial retardation plate having a refractive anisotropy nx>ny>nz be used to enhance the viewing angle properties of VA mode liquid crystal display device (as disclosed in Japanese Patent 3,330,574). The symbols nx, ny and nz represent the refractive index of the aforementioned retardation plate in X axis direction, Y axis direction and Z axis direction, respectively. The aforementioned X axis direction is an axis direction along which the aforementioned retardation plate shows the maximum refractive index in the in-plane direction. The aforementioned Y axis direction is an axis direction perpendicular to the aforementioned X axis direction in the aforementioned plane. The aforementioned Z axis direction indicates the thickness direction perpendicular to the aforementioned X axis direction and the aforementioned Y axis direction.
On the other hand, with the recent growing demand for liquid crystal television, various liquid crystal systems, including IPS and PCB modes, have shown improvements in display system (JP-A-9-211444, JP-A-11-316378, JP-A-2003-15134, JP-A-11-95208 and JP-A-10-54982).
However, these approaches merely allow the elimination of light leakage with respect to a certain wavelength range (e.g., green light in the vicinity of 550 nm) but don't take into account light leakage in other wavelength ranges (e.g., blue light in the vicinity of 450 nm, red light in the vicinity of 650 nm). Accordingly, these approaches leave something to be desired in the solution to so-called color shift, that is, coloration with blue or red developed when viewed obliquely during black display.